This invention relates to motor control systems and, more particularly, to a full wave motor control system for operating a DC motor.
Motor control systems are well known in the prior art for operating a DC motor from an AC supply at preselected speeds. Typically, such systems utilize phase control circuits connected between the DC motor and the cyclically varying AC power supply. A firing circuit is provided which responds to speed selection, typically via a potentiometer, for firing the phase control circuit at the appropriate phase angle of the power supply so as to control the motor speed. In particular, the phase control circuit typically includes a silicon controlled rectifier (SCR) serially connected between the power supply and the motor and the firing circuit includes a charging capacitor connected to the gate electrode of the SCR so as to trigger the SCR upon reaching a predetermined voltage. Speed control is then attained by controlling the charging rate of the capacitor. Such systems typically further include some feedback from the motor for purposes of regulating the speed so that the selected speed is maintained irrespective of variations in line voltage and/or motor load.
The aforedescribed arrangement provides half wave operation of the motor. Disadvantageously, such operation is noisy, especially at low motor speeds. It is therefore an object of the present invention to provide a full wave motor control system which results in smoother and quieter motor operation.
It would be desireable to be able to utilize the basic half wave circuitry in a full wave system because such circuitry has proven to be extremely reliable and cost effective. With standard half wave operation, the charging capacitor has sufficient time to be discharged and the SCR is turned off during the non-operative half cycle of the AC supply. However, with full wave operation there is no such non-operative time. It is therefore a further object of this invention to provide a full wave motor control system which utilizes a conventional half wave phase control circuit and insures that the charging capacitor is fully discharged and the SCR is turned off at the start of each half cycle of the AC supply.